1. Field of the Invention
The present invention relates to a magneto-optical recording-reproducing method comprising projecting a light beam onto a magneto-optical medium of a multi-layer structure having layers such as a memory layer and a displacement layer; displacing a domain wall of a record mark in the displacement layer without change of recorded data in the memory layer by utilizing a temperature gradient in a temperature distribution caused by light beam projection; and detecting a change of polarization direction of the reflected light beam to reproduce the record mark.
2. Related Background Art
Magneto-optical mediums for rewritable high-density recording are known which record information by forming magnetic domains in a magnetic thin film by thermal energy of a semiconductor laser and read out the information by utilizing a magneto-optical effect. In recent years, higher recording density of the magneto-optical medium is demanded. In an optical disk such as a magneto-optical medium, the linear recording density depends greatly on the laser wavelength and the numerical aperture of the objective lens of the reproducing optical system. Specifically, the laser wavelength .lambda. and the numerical aperture NA of the objective lens of the reproducing optical system decide the diameter of the beam waist, whereby the detectable range of the spatial frequency of record mark reproduction is limited to about 2NA/.lambda.. Therefore, for achieving higher recording density with a conventional optical disk, the laser wavelength should be shorter and the NA of the objective lens should be larger in the reproducing optical system. However, the improvements in the laser wavelength and the numerical aperture of the objective lens are limited naturally.
For a still higher recording density, the constitution of the recording medium and the reading method are being improved. For example, Japanese Patent Application Laid-Open No. 6-290496 discloses a signal-reproducing method and an apparatus therefor. This method records signals in a memory layer of a multi-layered film having a displacement layer and the memory layer coupled magnetically, and reproduces the record marks of less than the light diffraction limit of the optical system by displacing the domain walls of record marks in the displacement layer by utilizing a temperature gradient caused by irradiation of a heating light beam without changing recorded data in the memory layer, magnetizing uniformly and almost entirely the light beam-spotted region on the reproducing layer, and detecting the change of polarization direction of the reflected light beam. This method reproduces signals in a rectangle form as shown in FIG. 2E, which enables reproduction of record marks of a frequency of less than an optical diffraction limit of the optical system without decreasing the reproduction signal amplitude. Thereby, the medium and method for the magneto-optical recording are greatly improved in the recording density and the transfer speed.
FIG. 1 shows a constitution of a conventional system. In FIG. 1, magneto-optical disk 1 is constituted of substrate 2 magneto-optical layer 3 formed thereon, and protection layer 4 formed further thereon. Substrate 2 is formed from glass or a plastic material. Magneto-optical layer 3 is constituted of a multiple layer comprising at least a memory layer and a displacement layer, and is capable of reproducing record marks of less than an optical diffraction limit of the optical system by displacing a domain wall by utilizing a temperature gradient caused by light beam irradiation without changing recorded data in the memory layer, magnetizing uniformly and almost entirely the reproducing light beam-spotted region on the reproducing layer, and detecting the change of polarization direction of the reflected light beam. Magneto-optical disk 1 is set to a spindle motor by a magnet chucking or a like means to be rotatable on a rotation axis.
Parts 5 to 17 constitute an optical head for projecting a laser beam to magneto-optical disk 1 and for receiving information from reflected light. The parts comprise condenser lens 6 as an objective lens, actuator 5 for driving condenser lens 6, semiconductor laser 7 of a wavelength of 680 nm for record reproduction, semiconductor laser 8 of a wavelength of 1.3 .mu.m for heating, collimator lenses 9,10, dichroic mirror 11 for completely transmitting light of 680 nm and completely reflecting light of 1.3 .mu.m, beam splitter 12, dichroic mirror 13 for intercepting light of 1.3 .mu.m and completely transmitting light of 680 nm to prevent leakage of light of 1.3 .mu.m into the signal detecting system, .lambda./2 plate 14, polarized light beam splitter 15, photosensors 17, condenser lenses 16 for the photosensors, and differential amplification circuit 18 for differentially amplifying the condensed and detected signals for respective polarization direction.
The laser beams of 680 nm and 1.3 .mu.m emitted respectively from semiconductor lasers 7,8 for recording-reproducing and heating are introduced through collimator lenses 9,10, dichroic mirror 11, beam splitter 12, and condenser lens 6 to magneto-optical disk 1. Condenser lens 6 moves in the focusing direction and the tracking direction under control by actuator 5 to focus the laser beams successively on magneto-optical layer 3 by tracking along a guiding groove formed on magneto-optical disk 1. The light flux of 1.3 .mu.m is made smaller than the aperture diameter of condenser lens 6 to make the NA smaller than that of the light of 680 nm which is condensed through the entire area of the aperture. The heating spot, which is formed with a larger wavelength and a smaller NA, has a larger diameter of heating beam 74 than the recording-reproducing spot of recording-reproducing beam 73 as shown in FIGS. 3A and 3B. Thereby, a desired temperature gradient is produced in the recording-reproducing spot region on the moving disk face as shown in FIG. 3D. Numeral 75 indicates a Ts isotherm. The laser beam reflected by magneto-optical disk 1 is deflected by beam splitter 12 to the optical path toward polarized light beam splitter 15, and travels through dichroic mirror 13, .lambda./2 plate 14, and polarized light beam splitter 15. The split light beams are respectively condensed by lenses 16 onto sensors 17 corresponding to magnetization polarity of the spot on the magneto-optical layer. The condensed light beams are composed only of 680 nm light since dichroic mirror 13 intercepts the 1.3 .mu.m light. The outputs from the respective photosensors 17 are amplified differentially by differential amplifier 18 to output the magneto-optical signals. Controller 20 receives information on rotation rate of magneto-optical disk 1, recording radius, recording sectors, and so forth and outputs recording power, and recording signals to control LD driver (laser diode driver) 19, and magnetic head driver 24. LD driver 19 drives semiconductor lasers 7,8 In this example, LD driver 19 controls recording power, reproducing power, and heating beam power.
Magnetic head 23 applies a modulation magnetic field onto the laser irradiation spot on magneto-optical disk 1 for the recording operation. Magnetic head 23 is placed in opposition to condenser lens 6 with interposition of magneto-optical disk 1 During recording, recording-reproducing semiconductor laser 7 applies recording laser power by DC (direct current) light irradiation, and synchronously magnetic head 23 produces magnetic fields of different polarities under control by magnetic head driver 24 in correspondence with the recording signals. Magnetic head 23 moves with the optical head in a radius direction of the magneto-optical disk 1, and applies a magnetic field successively on recording onto the laser irradiation site of magneto-optical layer 3.
The recording-reproducing operation is explained by reference to FIGS. 2A to 2F. FIG. 2A shows recording signals, FIG. 2B a recording power, FIG. 2C modulating magnetic fields, FIG. 2D record marks, FIG. 2E reproducing signals, and FIG. 2F binary signals. In recording of the recording signals as shown in FIG. 2A, the power of the semiconductor laser is controlled to be at a prescribed level with the start of the recording operation, and a modulating magnetic field is applied in accordance with the recording signals. Thereby, record marks are formed in sequence in the process of cooling of the memory layer as shown in FIG. 2D, where the line-shadowed portions are magnetic domains magnetized in the direction corresponding to the record marks in the present invention, and the white blank portions are magnetic domains magnetized in the reverse direction thereto.
The portion of the guiding groove (hereinafter referred to as a groove) of the magneto-optical disk 1 is preliminarily annealed at a high temperature by projection of a laser beam to modify the portion of the recording medium layer so that the domain wall of the record mark will not form a closed loop, or a closed magnetic domain. This treatment facilitates the displacement of the domain walls.
The reproduction operation is explained below by reference to FIGS. 3A to 3D, wherein numeral 76 denotes a displacement layer, 77 a switching layer, 78 a memory layer, 99 a groove, 100 a land, and 70 a magnetic domain pattern of the displacement layer 76. The thin dotted line 122 represents the domain wall existing only in the memory layer. The displacement layer is heated by a heating beam 74 up to a temperature for causing the displacement of the domain wall in the displacement layer of the medium. The isotherm 75 of the temperature Ts of the recording medium, which is the main factor for inducing displacement of the domain wall, crosses the beam movement direction 71 in the forward portion and in the backward portion of the beam spot. The domain walls can displace backward from the front side and forward from the back side of the beam movement direction as shown by numeral 72 in FIG. 3A. Therefore, the magnetic displacement signals from the front side only can be detected by placing record-reproducing beam 73 only at the front side of the beam-moving direction as shown in FIG. 3A. Similarly, the magnetic displacement signals from the back side only can be detected by placing record-reproducing beam 73 at the back side of the beam-moving direction as shown in FIG. 3B.
In either case, the record mark sequence as shown in FIG. 2D is reproduced by the record-reproducing beam to obtain reproduced signals (FIG. 2E), and further to obtain binary signals (FIG. 2F). In the above magneto-optical recording-reproducing method, a light beam is projected to cause displacement of the domain walls of the record marks in the displacement layer by utilizing a temperature gradient caused by the light beam without a change of the recorded data in the memory layer, and the change of the polarization direction of the reflected light beam is detected to reproduce the record marks. According to this magneto-optical recording-reproducing method, the magnetization states carried by the reproducing beam are all equal as shown in FIGS. 3A and 3B. Therefore, the reproduced signals are rectangular, and record marks of less than a diffraction limit of the optical system can be reproduced without decreasing the reproducing signal amplitude. Thereby, a medium and a method for magneto-optical recording can be provided which have been improved in recording density and transfer rate.
However, the prior art described above has disadvantages of high cost owing to many optical parts for the heating laser beam, various adjustment steps in assemblage of the apparatus, and two laser systems. For solving the problems of the high cost, the heating and reproduction are required to be conducted with one light beam system.
In the case where the heating beam is not employed, the maximum temperature point in the high temperature region formed by light beam lies within the irradiation area of the light beam as shown in FIGS. 4A and 4B. The thin dotted line 122 represents the domain wall existing only in the memory layer in FIG. 4A. In this case, the reproduced signal is a synthesized signal formed from two signals: a signal generated by displacement of the domain wall at the front portion of the beam movement direction toward the maximum temperature point by the temperature gradient, detected at region 81: f(t) (f(t)=0 at t&lt;0) where t is a time after a start of read-out of the record mark sequence and t=0 means the read-out start time; and another signal generated by displacement of the domain wall of the rear portion of the beam movement direction toward the maximum temperature point by the temperature distribution, detected at region 82: .alpha..multidot.f(t-.beta.) (f(t)=0 at t&lt;0). Therefore, the synthesized signal is represented by h(t)=f(t)+.alpha..multidot.f(t-.beta.) where .alpha. is a coefficient for the read-out level for a signal from the front portion of the moving light beam, and .beta. is a time of delay of read-out.
For example, when the recorded signal sequence shown in FIG. 5A is reproduced, the recorded signals are read out, with the movement of the reproduction beam, through the states shown in FIGS. 5B1 to 5B4 to give reproduced signals as shown in FIG. 5CA, which is superposition of the signals generated by domain wall displacement from the front side (FIG. 5CB) and the signals generated by domain wall displacement from the back side (FIG. 5CC) of the optical beam. Therefore, in this case, the recorded information cannot be reproduced with a sufficient margin by a conventional technique of slice-leveling and binarizing the median of repeated reproduction signals of shortest marks disadvantageously.